How to change the useful space that bevy_egui uses?

[nuzzles]

I have a custom macOS titlebar, and I want egui to not use any of the space in my virtual titlebar, e.g. the top 28 pixels of space.

How do I tell egui that the screen rect is actually 28pixels smaller?

I tried something like this, it did not work:

        #[cfg(target_os = "macos")]
        app.add_systems(
            EguiPrimaryContextPass,
            macos_titlebar.run_if(woot_app::fullscreen::is_windowed),
        );

...

#[cfg(target_os = "macos")]
fn modify_screen_rect(mut contexts: EguiContexts) -> Result {
    let ctx = contexts.ctx_mut()?;

    // Modify the raw input to shrink the screen rect
    ctx.input_mut(|i| {
        let old_rect = i.screen_rect;
        i.screen_rect = egui::Rect {
            min: egui::Pos2 {
                x: old_rect.min.x,
                y: old_rect.min.y + MACOS_TITLEBAR_HEIGHT,
            },
            max: old_rect.max,
        };
    });

    Ok(())
}

[vladbat00]

Hi! You can try to render egui to a separate camera and limit the camera's viewport. You might find this example useful: https://github.com/vladbat00/bevy_egui/blob/main/examples/side_panel.rs
The only difference is that you'll need to edit the egui related camera (and only on setup), instead of the other one.

To make the multiple cameras setup work, note that you should disable automatic context creation:

bevy_egui/examples/side_panel.rs

Line 127 in 74fd9d9

egui_global_settings.auto_create_primary_context = false;

and initialise your Egui camera with the PrimaryEguiContext component and correct clear color and blend settings:

bevy_egui/examples/side_panel.rs

Lines 158 to 172 in 74fd9d9

	commands.spawn((
	// The `PrimaryEguiContext` component requires everything needed to render a primary context.
	PrimaryEguiContext,
	Camera2d,
	// Setting RenderLayers to none makes sure we won't render anything apart from the UI.
	RenderLayers::none(),
	Camera {
	order: 1,
	output_mode: bevy_camera::CameraOutputMode::Write {
	blend_state: Some(BlendState::ALPHA_BLENDING),
	clear_color: ClearColorConfig::None,
	},
	clear_color: ClearColorConfig::Custom(Color::NONE),
	..default()
	},